1. Field of the Invention
The present invention relates generally to motion damping of floating structures and, more particularly, to oil-and-gas drilling and production platforms using winches for deploying and pulling in one or more mooring cables.
2. Description of the Prior Art
A floating structure, for example, a drilling/production platform, is effectively a spring mass system. As such, it is subject to resonant oscillatory motions in response to wave and tidal actions in the seaway. There are six well-recognized motions: pitch, roll, heave, surge, sway, and yaw motions.
Resonant motion occurs when the natural period of a particular motion is substantially equal to the period of the wave, having substantial energy in the design seaway, which induces such motion in the platform.
The patent literature describes various structures and arrangements for dynamically and passively damping a floating platform. For example, the systems described by Bergman in U.S. Pat. No. 4,167,147 employ arrangements to create anti-heave forces that are in phase opposition and proportional to the heave velocity of the platform (Newtonian damping).
Bergman's damping system is intended to exert anti-heave forces when the vessel heaves up and also when it heaves down. These anti-heave forces are exerted on the vessel in a direction opposite to its vertical motion; they are much smaller than the actual wave forces which produce the heave; and they are intended to achieve a substantial decrease in heave amplitude, especially when the vessel is about to approach resonance.
Bergman illustrates in FIG. 14 a passive damping system which requires a tensioned flexible cable whose lower end is anchored to a weight on the sea floor. Its upper end passes over a sheave and is fixedly secured to the platform's upper deck. Also on the upper deck is a hydraulic cylinder whose piston rod supports the sheave. The cylinder is filled with pressurized oil below the piston. A restrictive orifice is interposed in the pipe between an oil supply reservoir and the cylinder to restrict the oil flow between the cylinder and the reservoir.
In deeper waters, the platform's motion responses to wave action, in particular its heave, becomes a serious problem for the production risers which are suspended by dynamic tensioning devices having a fixed stroke range.
For example, a deep-floating production platform, described in copending application Ser. No. 07/239,813, assigned to the same assignee, and now Pat. No. 4,850,744, produces oil through wellheads suspended above the waterline together with their production risers. A dynamic riser tensioner is required to ensure that each riser is constantly subjected to a constant tension. Such a deep-drafted production platform makes use of a spread-type mooring line system for anchoring the structure to the seabed. An individual winch, or the equivalent thereof, is available on the platform structure for deploying and pulling in each mooring cable.